That totally depends on your workload. In my case my working set happens to be mostly equal to the most recently inserted data. If you have to regularly access lots of random documents with no locality whatsoever then your working set is very large and should fit in memory.
By sticking with the default _id, with my workload my _id index doesn't have to fit into memory. I can't actually use the default _id for various reasons but that's a whole different discussion.
